Get Cluster "Distances"

Input a SAHN tree (e.g. the result of a call to linkage) and the number, n, of clusters into which the SAHN tree is to be partitioned (e.g. by the function cluster). Output is a list of distances where the i'th distance corresponds to cluster i as produced by a call to cluster to produce n clusters. Each "distance" is the number associated with the node at the "root" of each cluster, i.e. the distance between the final two sub-clusters merged to produce the i'th cluster.